The present invention relates to communications systems, and, more particularly, to a self-synchronizing demodulator for frequency division multiple access signals.
Communications systems for digital data often use a frequency division multiple access (FDMA) approach to transmitting data from different sources or users. In the FDMA approach, each user channel is assigned a discrete portion of the transmitted frequency spectrum so that many channels can be transmitted over a single transmission medium. In one approach, phase shift keying (PSK) techniques, including quadrature phase shift keying (QPSK) techniques, are employed in which a constant amplitude carrier has its phase angle varied to encode digital information.
In the past, the transmitted information has been recovered at a receiver using a separate PSK demodulator for each of the FDMA channels to provide individually accessible streams of data associated with each source.
The use of separate PSK demodulators to accommodate each of the FDMA channels, while adequate for some purposes, is impractical for applications where a receiver's weight and power are critical factors, for example, in regenerative satellites. This is especially true for a system is designed to process hundreds or thousands of channels.
More recently developed alternatives use a single receiver for multiple channels. The FDMA signals are typically transformed using a digital fast Fourier transform or a wide bandwidth surface acoustic wave (SAW) Fourier transform processor for transforming the FDMA signals into a time division multiplexed (TDM) signal of modulated data. The resulting TDM signal is demultiplexed to provide individually accessible streams of demodulated data.
In order for a single demodulator to effectively process multiple channels of data, the channels must be synchronized. In one approach, individual user stations adjust timing in response to error signals from a satellite or other central procession station. The feedback can be formed by dithering a symbol window back and forth to determine whether the symbol segments of each channel are early, late or on time. This technique involves sensing the amplitude of each demodulated symbol segment, comparing successive amplitudes corresponding to the given channel to form a discriminate, and generating a timing error signal.
This timing error signal is delivered to the transmitting source using multiplexing techniques. The users thus can adjust the data transmission timing of their communications channel in order to achieve collective synchronization of the symbol segments.
However, there are disadvantages to this approach to synchronization. In a satellite-based system, the number of user stations can be many times the number of channels, since it is expected that only a percentage of the stations need to access the satellite at any given time. Furthermore, these stations are located at the "consumer" end of the satellite communications system, where cost and simplicity are critical. Synchronization of user stations requires each user station to include means for receiving and interpreting the error signals, and means to adjust timing. Furthermore, the round trip of the error signal followed by a corrected transmission signal introduces a time delay during which communications must continued impaired. Finally, the error signal consumes valuable communications bandwidth.
One object of the present invention is to synchronize FDMA channels centrally, e.g., on board a communications satellite. Thus providing the advantages of a single demodulator system without the need for user-based synchronization.